From the past, semiconductor pressure sensors are used for measuring the pressure of intake and exhaust gases in automotive engines, the pressure of exhaust gases in motorcycle engines, and the like. For instance, pressure sensors for measuring the engine intake pressure in automobiles generally use semiconductor pressure sensor chips utilizing piezoresistance effect as the pressure detecting device. Semiconductor pressure sensor chips of this type are designed to detect as an electrical signal a change of resistance in response to any deformation of a diaphragm made of piezoresistive material. The semiconductor pressure sensor has a semiconductor pressure sensor chip mounted within a cavity in a resinous housing. For example, the semiconductor pressure sensor chip is mounted within the housing cavity with the aid of an adhesive directly or after placement on a glass pedestal. Bonding wires are used to provide electrical connection between the sensor chip and leads which are embedded in the resinous housing by insert molding.
The semiconductor pressure sensors of this type are not only subject to varying pressure and varying temperature, but are also exposed to an environment of gasoline vapor, water vapor, acidic exhaust gases or the like. It is thus essential to fill and seal the cavity interior with an electrically insulating fluorochemical gel material for the purpose of protecting the semiconductor pressure sensors from electrical, mechanical, thermal and chemical attacks.
The semiconductor pressure sensors encapsulated and protected with fluorochemical gel materials include sensors using fluorosilicone gel materials as disclosed in Japanese Patent No. 2,525,433 and JP-A 2001-153746 and sensors using perfluoropolyether gel materials as disclosed in JP-A 2001-99737, JP-A 2001-153746, JP-A 2001-304999 and JP-A 2001-311673. Of these, JP-A 2001-153746 proposes the use of a gel material having a degree of saturation swelling in gasoline at 20° C. of up to 7 wt % and a penetration of 10 to 30 as measured by the consistency test of JIS K2220 using a ¼ cone, as a means of preventing bubbles from generating from within a gel material under negative pressure or at elevated temperature. Also, JP-A 2001-304999 proposes the use of a gel or rubber material having a glass transition temperature of up to −30° C., as a means of preventing degradation of sensor performance by an increased stress in a low-temperature environment and lowering of chemical resistance.
However, in the case of the above semiconductor pressure sensor encapsulated with a fluorochemical gel material having a low penetration as well as the above-described properties, the generation of bubbles from within the gel material can be restrained whereas the gel material will separate partially from the surface of gold-plated leads, the cavity wall of a resinous housing or the like, especially due to short adhesion of the gel material under negative pressure or at low temperature (below 0° C.). Then condensed liquid ingredients will accumulate at the separated portions, becoming a source of bubble generation. Bubbles generated therefrom will grow or travel with a change of temperature or pressure and cause cracking to the gel material or breakage to the bonding wires. Then not only the insulating/protecting function, but the sensor function itself is lost. On the other hand, even in the case of the semiconductor pressure sensor encapsulated with a fluorochemical gel material having a degree of saturation swelling in gasoline at 20° C. of up to 7 wt % and a Tg of up to −30° C., if the gel material has a high penetration, the gel material, when subjected to pressure cycling in a low-temperature state below 0° C. over a long period of time, undergoes initial bubble formation and eventual liquefaction, resulting in a portion thereof flowing out of the cavity and interfering with the insulating/protecting function.